1. Field of the Invention
The present invention relates generally to a 2-methyl-1,3-propanediol-based polyester film having substantial shrink properties, and the polyester resin from which the film is formed. More specifically, it is related to a modified polyethylene terephthalate in which 2-methyl-1,3-propanediol is partially substituted for ethylene glycol. The degree of shrinkage and the degree of crystallinity in the resulting film can be varied and controlled by varying the relative amounts of 2-methyl-1,3-propanediol in the resin and film.
2. Description of Related Art
Shrink films are known in the art. Such films are useful for many applications, including shrink-to-fit labels for bottles and other dimensional items. Polyester-based shrink films are particularly advantageous for use on polyester containers, as they facilitate the recycling process by allowing polyester containers to be reclaimed along with their polyester labels without introducing incompatible resins into the recycling stream.
When shrink films are used in labeling applications, they are typically printed with text, images and the like. Thus, it is preferred that shrink films be printable, either directly, or through compatibility with print adhesion promoting coatings or treatments. For many applications, the shrink film is preferably clear or transparent. It is preferably durable under normal packaging, transport and storage conditions. The film should optimally be cost effective to manufacture, and should minimize the use of organic solvents or other potentially environmentally hazardous compounds in its manufacture and use, to the extent possible. Films adapted for in-line coating application are also desirable. A film providing a combination of these benefits is desirable.
Known shrink films include films based on PVC (poly(vinyl chloride)), oriented polystyrene, as well as more environmentally sensitive modified copolyesters such as PETG (1,4-cyclohexanedimethanol modified poly(ethylene terephthalate)). These films provide excellent shrink properties and provide good strength and consistent appearance. However, more cost-effective alternatives to PETG-based and other known shrink film resins are desirable. In addition, shrink film resins that are more conducive to recycling, and that can be processed with fewer modifications on conventional PET film manufacturing equipment are also desired.
Certain diol-based polyester resins have been suggested for use in shrink films. See, e.g., U.S. Pat. No. 4,996,291 to Yoshinaka et al. However, the numerous propanediols disclosed in the Yoshinaka et al. patent do not include the structurally simpler 2-methyl-1,3-propanediol, and instead are all more highly branched propanediol-containing copolymers using monomers such as 2-methyl, 2-ethyl-1,3-propanediol and 2-methyl-2-propyl-1,3-propanediol. The utility of 2-methyl-1,3-propanediol was clearly not appreciated, and in fact expressly discounted through omission by Yoshinaka et al. from an otherwise extensive list of propanediols. Moreover, no commercial film embodying the teachings of the Yoshinaka et al. patent is known to be available, suggesting technical limitations and other difficulties in utilizing the diols listed therein to create desirable shrink films.